Many industrial and commercial processes involve the generation of combustible dust ranging from grain elevators to various types of dust collectors. Particle sizes smaller than 100 microns will tend to float in air, and when concentrations of these particles, beyond certain minimum amounts, are suspended in air, the environment in which the mixture exists needs only a source of ignition to produce an explosion.
Means have been developed to reduce the destructive force of explosions through venting but this is feasible only when the process equipment is located out of doors or close to exterior walls or ceilings, through which release of flaming materials would have no adverse consequences.
Explosion suppression techniques have also been developed which overcome some of the shortcomings of venting systems. As with all safety systems, however, one of the most important elements of an explosion suppression system is a reliable means of detecting a developing hazard. Among the options for such detection are optical detectors, heat sensors and pressure responsive devices. Optical devices tend to become obscured by dusty environments and heat sensors are generally too slow for use with rapidly developing fires. For hazards involving dusts, the most practical method of detecting a developing explosion is through the use of pressure sensors.
Pressure sensors generally employ a flexible diaphragm to convert pressure changes into mechanical or electrical output that can be used for a variety of purposes ranging from process equipment control to actuation of explosion suppression apparatus. To survive in a process environment and maintain response characteristics, it is necessary not only for the sensor mechanism itself to remain functional, but also for the sensor diaphragm to maintain its ability to sense pressure changes. Finally, the pressure passages between the sensor diaphragm and the process environment must be sufficiently clear to transmit pressure changes.
Because explosions involving dust/air mixtures develop rapidly and can become very destructive if not arrested at an early stage, the actuation threshold for pressure sensor must be quite small, for example 0.3 psig. This threshold must be maintained throughout the life of the system. Commonly employed technology requires the sensors to be periodically removed from their point of application for threshold measurement and adjustment. This is an expensive, cumbersome procedure and requires availability of spare sensors or requires system shutdown or resulting periods of no protection.
In addition to the problem of assuring accuracy of pressure actuation thresholds, many filter processes operate at negative pressures that vary as dust builds up on filters or due to the process itself, such as grain transfer from an enclosure.
There is clearly a need, not only for means of verifying and maintaining the threshold setting of pressure sensors, but also of compensation for negative process pressures that would prevent timely actuation of such pressure sensors i.e. actuation during fire-ball generation. Further, this verification and compensation must be applied in such a way that false actuation of the system is minimized or eliminated.
It is well known in the prior art to arrange a pair of sensing devices so that both must indicate the presence of a hazard before actuation of a safety system. The object of such an arrangement is to reduce the likelihood that false response of one sensor will cause false actuation of the safety system.
While such an arrangement can reduce the incidence of false actuation, it has the disadvantage that failure of one sensor is automatically a failure of both. The advent of solid state logic devices has made it practical to connect sensors into "voting" arrangements. Using three sensors, it is possible to require response from any two sensors for safety system actuation.
It is therefore an object of the Applicant's invention to provide a hazard response structure which will eliminate false discharge of suppression material by requiring multiple response to hazard existence.
It is a further object of the Applicant's invention to provide a means for measuring the actuation threshold of pressure sensors and for adjusting the same without removal of the sensors from their sensing stations.
It is a further object of the Applicant's invention to provide for continuous and automatic monitoring of pressure sensor circuits by employing normally closed switch contacts and to provide the same in a protective housing so as to avoid accidental shorting of the circuits.
It is still a further object of the Applicant's invention to provide a pressure sensor system including at least three sensor units with a control logic circuit that requires response from any two of the sensors before initiation of a suppression system.
It is still a further object of the Applicant's invention to provide a safeguard circuit for a pressure response, suppression material actuation or delivery system which includes an isolated, voltage detection means which will prevent actuation of the suppression system during connecting or disconnection of the system to and from a power source due to power line failure or interruption by personnel for system maintenance, and which will require absolute voltage reversal of the system to achieve suppression system actuation.
These and other objects and advantages of the Applicant's invention will more fully appear from a consideration of the accompanying drawings and disclosure.